1. Field of the Invention
The invention pertains generally to computer circuits. In particular, it pertains to charge pump circuits.
2. Description of the Related Art
A charge pump circuit is a circuit that can produce an output voltage that is higher than the circuit""s supply voltage. This is accomplished in a series of stages, with each stage increasing the voltage from its input to its output by moving charge in that direction, with the output providing the input of the next stage. By cascading enough stages in this manner, an output voltage that is several times the circuit""s supply voltage can be produced.
Charge pump circuits can be used to increase voltage in both the positive and negative directions. For example, a positive charge pump circuit with a supply voltage of Vcc=+3 volts might be used to produce an output voltage of approximately +10 volts, while a negative charge pump circuit with the same Vcc supply voltage might be used to produce an output voltage of approximately xe2x88x9210 volts by moving charge in the opposite direction. FIG. 1 shows a schematic for a single stage 1 of a negative charge pump circuit, including charge pump cell 10, in which the clock signals CLK1 and CLK2 that alternate between Vcc and 0 volts are used to create an output voltage at output 13 that is more negative than the input voltage at input 11. Because the voltage difference across a capacitor cannot change instantaneously, CLK1 causes the voltage at input 11 to be alternately higher and lower than at output 13, while CLK2 turns on the switchable diode comprised of transistors T1-T3 only when input 11 has a higher voltage than output 13. This operation causes charge to flow from the input to the output, but prevents it from flowing in the opposite direction. The input of a following stage (not shown) will have its input connected to output 13, and will also have two similar clocks that are timed with CLK1 and CLK2 to assure that the charge flows into that following stage from charge pump stage 1. The current flow and voltage buildup in the negative direction occurs from right to left in FIG. 1. If this were a positive charge pump, the schematic would look basically the same, but the current flow and voltage buildup would proceed from left to right, and the relative timing (not shown) between the clock signals would be different. The basic operation of positive and negative charge pump circuits is well known, and will not be discussed further.
The relatively high voltages in the latter stages of a multi-stage charge pump circuit can cause stress and eventual failure in the semiconductor material if those voltages are allowed to remain too long. Because of the likelihood of stress and damage as the part undergoes operation cycles over time, the capacitors of a charge pump are discharged after the pump is turned off to prevent retention of these high voltages for long periods of time. This process of discharging is called xe2x80x98initializationxe2x80x99 and is typically controlled by a portion of the charge pump shown in FIG. 1 as initialization circuit 12, which turns on transistors T5, T4 that are connected between nodes 1, 2 and ground. In a positive charge pump, a positive voltage is being discharged by this initialization circuit. Since only positive voltages are involved, the discharge transistor can be controlled by a positive logic signal, which is easily derived in the surrounding control logic that is operating between Vcc and ground.
However, with a negative charge pump a high negative voltage must be discharged by initialization circuit 12, and this requires a negative control signal to turn on the negatively biased discharge transistors T4, T5. Transistors in conventional charge pumps do not use triple well technology (where multiple nested xe2x80x98wellsxe2x80x99 of alternately doped semiconductor material are used to electrically isolate the transistor from the underlying substrate). They cannot use an n-type transistor to transfer a negative voltage, as that would forward bias the diode between the n+ doping and the to pxe2x88x92 substrate. Therefore, conventional negative charge pumps and the associated control circuitry use p-type devices only. The necessary negative control signal is created by implementing another negative charge pump circuit 14, with a switchable output, whose sole purpose is to generate a negative voltage to operate the negatively-biased discharge transistors T4, T5. Charge pumps require a relatively large amount of die space, and this initialization charge pump circuit 14 uses up valuable space on the die that could otherwise be used for additional flash memory capacity or other important functions.
Another method of initialization involves discharging the node to Vcc instead of ground, which eliminates the need for the additional negative charge pump. However, this requires additional time to charge the pump when operation is first started, since the nodes have to charge down from Vcc instead of ground. When used in flash memory, this slows down the overall speed of the erase operation.